select distinct ta.name from ta, tb, lnk where ta.id=lnk.ia and tb.id=lnk.ib and (tb.name='b1' or tb.name='b4');

returns 
a1
a2                        

insert into lnk values(1,1); --a1, b1
insert into lnk values(1,2);
insert into lnk values(1,3);
insert into lnk values(2,1); --a2, b1
insert into lnk values(2,4); --a2, b4
insert into lnk values(2,5);
insert into lnk values(2,6);